Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser diodes at the transmitter. Each diode generates the signal associated with one of the channels in the WDM signal. These laser diodes are tuned to operate at different wavelengths. When combined in an optical fiber, the WDM optical signal comprises a corresponding number of spectrally separated channels within a signal band. At the receiving end, the channels are usually separated from each other using thin film filter systems, to thereby enable detection by separate photodiodes.
WDM technology enables the collective amplification of the various channels in gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman pumping scheme. Other WDM applications include the dynamic routing of individual channels in optical WDM networks with multiple network access nodes.
In commercially available and proposed WDM systems, the channel assignments/spacings can be tight, 100 GigaHertz (GHz) to 50 GHz, based on the ITU grid. Further, the number of potential channels on a link can be large. Observation of the ITU Grid suggests 100""s of channels on a link in the Lxcex1, Cxcex1, and Sxcex1 bands, even if the 50 GHz offset of the Lxcex2, Cxcex2, and Sxcex2 band is ignored. Still other systems are being proposed that have assignments/spacings in the 10 to 20 GHz range. Thus, each channel must be confined to its channel slot frequency assignment to an absolute accuracy of less than 10 GHz, in some cases.
In order to verify the proper operation of these WDM systems, optical channel monitors are required. These devices typically have a tunable band pass filter that is scanned across the signal band to detect the individual channels. It can thus verify that proper guard bands are being maintained between adjacent channels. They can also be used to verify that the channel powers are consistent with each other such that one channel is not broadcasting with a power that is overwhelming adjacent channels.
In some applications, it is further desirable to have the ability to resolve the absolute wavelengths of the channels. This typically requires some sort of reference signal. Although some systems use capacitive sensing to determine the pass band of the tunable filter, other typically more accurate systems rely on optical reference signals. The filter is scanned across a reference signal with a known and highly stable spectral feature, such as a line of a distributed Bragg reflector laser. This is used to calibrate the tunable laser for a subsequent scan across the signal band of the WDM signal. From this information, the channel monitoring system either extrapolates or interrelates the absolute frequency scale in the WDM signal from the spectral feature in the reference signal.
The present invention is directed to a tunable filter system, which is preferably used as an optical channel monitor in a WDM system, although it has applicability in any tunable filter application requiring a wavelength reference.
In general, according to one aspect, the invention features a tunable filter system. This system comprises a signal source providing a WDM signal having multiple channels within a spectral signal band. A reference source generates a reference signal with spectral reference features that are typically located in a spectral reference band that is outside the signal band. A tunable filter is provided comprising a cavity bounded by at least two reflectors; at least one of these reflectors is a deflecting membrane to thereby create a tunable spectral pass band. The filter has a free spectral range that is greater than a combined bandwidth of the signal band and the reference band. The tunable filter pass band filters the reference signal and the WDM signal.
In the preferred embodiment, a filter controller is provided that tunes the pass band of the tunable filter successfully across the reference band and the signal band. In the current implementation, the controller passband first crosses the reference band, and then crosses the signal band.
A detector, detecting the beam that is filtered by the tunable filter provides a signal to the controller, which determines an absolute wavelength of the multiple channels within the signal band in response to the reference features in the reference band. In one implementation, the reference signal is generated from a broadband source and a fixed wavelength etalon. A beam combiner is then used to create a combined beam, including the WDM signal that is launched into the tunable filter.
In general, according to another aspect, the invention also features a WDM signal analysis method. This method comprises receiving a WDM signal having multiple channels within a spectral signal band and generating a reference signal having a spectral feature located in a spectral reference band that is outside the signal band. A launch beam is generated by combining the WDM signal and the reference signal. A tunable filter passband is tuned successively across the reference band and the signal band. With this information, the absolute wavelength of multiple channels in the signal band can be determined in response to the reference features in the reference band.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.